chassis design - github pageskjlaw.github.io/.../team_documents/2007row_chassis.pdf · chassis...

Chassis Design

Zan HechtManchester, NHJan 5th, 2007

2007 FIRST Rookie Workshop

Outline (modified)

• Basic Robot Design Theory• Building a Chassis• Building a Driveline• What’s in the KOP?


• What’s in the KOP?• Moving from VEX to FRC• Final Advice• Questions?

Basic Robot Design Theory


Skid (Tank) Steering


ROBOT

A B


Skid (Tank) Steering


A B


Steering Suggestions• Skid steering is easy

• Single-joystick controls are great for new drivers

• Two-joystick controls gives drivers more control


• Two-joystick controls gives drivers more control


4 Wheels vs. 2 Wheels



Slicks vs. Grips



Wheel Suggestions• It doesn’t matter how many wheels you have, as long as they all are driven

•If you plan to turn, you should only have two “grippy” tires

•Incline Conveyor Belt (wedge-top, rough-top)


•Pneumatic Tires

•Soft Rubber Tires

• Remaining wheels should be slick

•Hard rubber or plastic

•Omni-wheel/Wonder-wheel

•Zip ties (in case of emergency only!)


Center of Gravity



Weight Distribution Suggestions• Your center of gravity must be between your wheels

• Your center of gravity must be between your wheels even when your robot is at an angle


when your robot is at an angle

• The wheels closest to your center of gravity should be grippy


Chain Theory

16 32


16 32


?*inputoutput rpmrpm =

Chain Theory


?*

?*

inputoutput

inputoutput

torquetorque

rpmrpm

=

=


* inputinputoutput teeth

teethrpmrpm =

Chain Theory


?*

*

inputoutput

outputinputoutput

torquetorque

teethrpmrpm

=

=


output

inputinputoutput teeth

teethrpmrpm *=

Chain Theory


input

outputinputoutput

output

teeth

teethtorquetorque

teeth

*=


?=speed

Robot Speed


?=robotspeed


πwheelrpm=

Robot Speed


π**60 wheelwheel

robot Diameterrpm

speed =

Basic Robot Design TheoryMotor Performance Data

Speed (RPMs)Torque (oz.

in.)Current (Amps)

Power Out (Watts)

Efficiency Heat (Watts)

170 0.00 0.1 0.0 0% 1

159 4.68 0.3 0.5 26% 2

147 9.35 0.4 1.0 34% 2

136 14.03 0.5 1.4 36% 2


125 18.71 0.6 1.7 36% 3

113 23.38 0.7 2.0 34% 4

102 28.06 0.8 2.1 32% 4

91 32.73 0.9 2.2 29% 5

79 37.41 1.0 2.2 26% 6

68 42.09 1.1 2.1 23% 7

57 46.76 1.2 2.0 19% 8


Robot Speed

What size wheel should I use if I want my robot’s maximum speed to be 3 feet per second?


maximum speed to be 3 feet per second?


Robot Speed




3~**60120~

3 wheelDiameter=


Robot Speed




21

≈wheelDiameter (6 inches)


Robot Speed

If the 6” wheels are the largest I can fit onto my robot, how would I make my robot’s maximum


robot, how would I make my robot’s maximum speed 6 feet per second?


Robot Speed

If the 6” wheels are the largest in the kit, how would I make my robot’s maximum speed 6


would I make my robot’s maximum speed 6 feet per second (without damaging the motor

or making custom wheels)?

Put a sprocket on the motor that is half the size of the sprocket on the wheel.


Sprockets vs. Gears


Sprocket Gears


Sprockets vs. Gears



Sprockets vs. Gears


Maximum ratio 8:1

9–72 teeth

Infinite Ratio Possible

13 – ∞ teeth (<18 not recommended)


Sprockets vs. Gears


Face Alignment Critical Spacing Critical


Gear and Sprocket Recommendations

• Sprockets are used with chains, gears mesh with each other

• Sprockets and gears are NOT interchangeable


• Sprocket and chain systems are easier to build than gear systems

• Gear systems can be smaller and lighter than chains and sprockets


Idler Gears

16 3232


16 3232


Idler Gears

16 3216


16 3216


Further Gear and Sprocket Recommendations

• Idler gears change direction of motion, but don’t change gear ratio

• Properly designed gear or chain and sprocket systems are ~97% efficient at each gear/sprocket, so idlers don’t effect much if you don’t go overboard


each gear/sprocket, so idlers don’t effect much if you don’t go overboard


Wheelbase



Wheelbase Recommendations

• Short and wide robots turn easily and have lots of control, but will tend to not drive straight

•Long and narrow robots will not turn easily and will have poor turning


•Long and narrow robots will not turn easily and will have poor turning control, but will tend to drive very straight

•Depending on the task, you should balance the two

Building a Chassis

Building a ChassisDesign Tradeoffs

•Stable vs. Maneuverable

•Accessible vs. Compact


•Accessible vs. Compact

•Strong & Rigid vs. Light

•Manufacturable & Affordable vs. Everything

Building a ChassisKit Chassis


• Advantages: lightweight, quick to build, uses standard parts• Disadvantages: may not fit your design, requires added structure (that

will most likely be put on anyway)

Building a ChassisT-Slot Extrusion (80/20)


• Advantages: quick to build, standard parts, easy to create tension and to add fastening points

• Disadvantages: heavy, expensive

• Advantages: lightweight, strength, fits your design

• Disadvantages: takes time, requires skill,

Building a ChassisAluminum Tube and Plate


time, requires skill, non standard parts

Building a ChassisMiscellaneous


• Advantages: fits your design, unique• Disadvantages: takes much time, requires skill, non standard

parts

• Aluminum Extrusion– 1/16” – 1/8”: usable but will dent and bend– T-slot: use 1” sized profiles or higher

• Aluminum Plate, Bar, and Angle– 3/16” – ¼” used often

• Plastic Sheet

Building a ChassisMaterials


• Plastic Sheet– Spans structures, provides bracing– Polycarbonate (LEXAN, etc.) NOT Acrylic (Plexiglas, etc.)

• Wood– Lightweight and easy to use– Will splinter and fail but can be fixed

• Steel Tube and Angle– Strong, but heavy, 1/16” wall thickness is plenty strong

• Misc– Extruded fiberglass, PVC tubing, etc. Use your imagination!

Building a Driveline

Building a DrivelineDesign Tradeoffs

•Speed vs. Power

•Traction vs. Maneuverability


•Traction vs. Maneuverability

Building a Driveline6-Wheel Drive


Building a DrivelineSwerve Drive


Building a DrivelineTreads


Building a DrivelineOther Wheel Configurations


Building a DrivelineStandard 4-wheel Tank Drive


Building a DrivelineWheel Sources

• Kit of Parts Skyway wheels (more available at FIRST team discount from 800-332-3357)

• Colson Casters (available from many places, including http://www.robotmarketplace.com/)


• FIRST Specific wheels (high traction wheels, omniwheels, etc)

•http://andymark.biz/•http://ifirobotics.com/

•Make your own (can be made from aluminum, wood, HDPE, lexan, etc.)


Driveline Recommendations

• There are many types of drivelines, choose the one that best fits your specific game strategy.

• A well driven, reliable, “vanilla” driveline will beat a complex and unreliable


• A well driven, reliable, “vanilla” driveline will beat a complex and unreliable driveline in competition.


Chain Wrap

Motor


Motor

WheelWheel


Chain Wrap

Idler


Motor

WheelWheel


Chain Tension



Further Gear and Sprocket Recommendations

• All sprockets must have >120º of chain wrap (180º is better)

• Chains “stretch” as they wear, have a way to adjust tension



Supporting Shafts


motor

bearingsprocket


Supporting Shafts


motor

wheel


Supporting Shafts


motor


Supporting Shafts

motor


motor


Supporting Shafts



Supporting Shafts

ONLY IF ONLY IF SHAFT IS SHAFT IS


SHAFT IS SHAFT IS SHORTSHORT


Shaft Support Recommendations

• Never side-load your motors – they’re not designed for it. Always have at least one bearing on the output, and try to have two whenever possible.

• If your shaft is supporting weight, support it in two places.


• If your shaft is supporting weight, support it in two places.

• Try to avoid supporting a shaft in three or more places – a misalignment will lead to a loss of power.

Questions?

chassis design - github pageskjlaw.github.io/.../team_documents/2007row_chassis.pdf · chassis...

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